Abstract:

A fluid-cooled mold for the production of a quartz crucible provided in
its interior with a space for flowing of cooling fluid comprises an outer
mold section made from a heat-conductive metal or alloy material and an
inner mold section closely arranged to an inner surface of the outer mold
section and made from a heat-resistant material.

Claims:

1. A fluid-cooled mold for the production of a quartz crucible provided in
its interior with a space for flowing of a cooling fluid, which comprises
an outer mold section made from a heat-conductive metal or alloy material
and an inner mold section closely arranged to an inner surface of the
outer mold section and made from a heat-resistant material.

2. A fluid-cooled mold according to claim 1, wherein vent-holes opening to
an inner surface of the mold are formed in the inner mold section.

3. A fluid-cooled mold according to claim 1, wherein the inner mold
section is integrally constituted with a plurality of split segments
comprising a dish-shaped lower split segment placed on a bottom portion
of the outer mold section and at least one ring-shaped upper split
segment detachably piled on the lower split segment and closely arranged
to an inner surface of the outer mold section.

4. A fluid-cooled mold according to claim 3, wherein each of the plural
split segments is made from a carbon material.

5. A fluid-cooled mold according to claim 1, wherein the outer mold
section is constituted as a fluid-cooling jacket.

6. A fluid-cooled mold according to claim 1, wherein the mold has a
bottomed cylindrical shape with an upward opening portion and is used for
producing a quartz crucible by a rotating mold method in which the mold
is rotated around a virtual line passing from a central position of the
bottom portion to a central position of the opening portion as an axis of
rotation.

Description:

BACKGROUND

[0001]1. Field of the Invention

[0002]This invention relates to a fluid-cooled mold for use in the
production of a quartz crucible, and more particularly to a fluid-cooled
mold having an optimized mold structure.

[0003]2. Description of the Related Art

[0004]A quartz glass crucible is used for pulling a silicon single crystal
as a semiconductor material, a silicon crystal as a solar battery
material or the like. For example, the silicon single crystal is mainly
produced by a method in which a polycrystalline silicon lump charged in
the quartz glass crucible is melted by heating to form a silicon melt and
then a seed crystal is immersed in the silicon melt and pulled therefrom.
The silicon crystal as a solar battery material is low in
single-crystallinity as compared with single silicon crystal as a
semiconductor material, but is produced by the same pulling method.

[0005]As a method of producing a quartz glass crucible is known a rotating
mold method. In this method, a rotatable bowl-shaped mold is used, and
raw quartz powder is deposited on an inner surface of the mold at a given
thickness along the inner surface by utilizing centrifugal force
generated during the rotation of the mold. Subsequently, the quartz
powder is melted and vitrified by heating to a temperature (about
2000° C.) higher than a melting point (melting temperature)
through arc discharge of an electrode(s) disposed above the mold and
around the rotation central axis of the mold to form a glass crucible
having a form along the inner surface of the mold, and thereafter the
resulting glass crucible is cooled and taken out from the mold.

[0006]A fluid-cooled mold has hitherto been known as a mold used for
producing the quartz glass crucible by the rotating mold method. For
example, JP-A-H11-43394 discloses that raw quartz powder is charged into
a rotating stainless-steel fluid-cooled mold and melted through arc
discharge to produce a quartz glass crucible. Similarly, JP-A-2002-154890
discloses that quartz powder is charged into a rotating stainless-steel
fluid-cooled mold and shaped through arc discharge under a reduced
pressure to produce a quartz glass crucible.

[0007]In the conventional stainless-steel fluid-cooled mold, the heat
damage of the mold is prevented by forming a space for flowing of a
cooling fluid inside a bottom portion and peripheral wall portion of the
mold and cooling the bottom portion and peripheral wall portion of the
mold with the cooling fluid under an environment by heating at a higher
temperature. The inner surface of the mold is usually cooled down to
about 100° C. On the other hand, the raw quartz powder deposited
on the inner surface of the mold is melted and vitrified by heating to a
temperature above the melting point through arc discharge.

[0008]In the cooling structure of the conventional fluid-cooled mold, the
quartz powder deposited on the inner surface of the mold is melted and
vitrified by heating above the melting point at an outer surface side
opposite to the inner surface of the mold, while a large portion of the
quartz powder located at the inner surface side of the mold remains at an
unmelted state without vitrification because heat in such a portion is
removed by the cooling of the mold. In order to form a vitrified layer
having a target thickness with the conventional fluid-cooling mold, the
quartz powder is used in a quantity larger by about 2 times than the
weight required for target thickness. Therefore, there is a problem that
the quantity of quartz powder for the formation of a product is small as
compared with the quantity used and the loss in the quantity of the
quartz powder is large.

[0009]On the other hand, there is known a carbon mold, the whole portion
of which is made of carbon. Although carbon is high in heat resistance as
compared with stainless steel, it is subjected to heat damage due to
oxidative consumption if used at a high temperature for an extended
period of time. There is a problem that the heat-damaged mold causes
failures in the shape of the quartz glass crucible and thus the whole of
the expensive mold is replaced with a new mold, which significantly
reduces the economic efficiency.

SUMMARY

[0010]This summary is provided to introduce a selection of concepts in a
simplified form that are further described below in the Detailed
Description. This summary is not intended to identify key features of the
claimed subject matter, nor is it intended to be used as an aid in
determining the scope of the claimed subject matter.

[0011]The invention is to solve the aforementioned problems in the
conventional fluid-cooled mold, and provides a fluid-cooled mold having
sufficient heat resistance and significantly reducing unmelted portions
of raw quartz powder by optimizing the structure of the fluid-cooled
mold.

[0012]The invention provides a fluid-cooled mold having the following
construction: [0013](1) A fluid-cooled mold for the production of a
quartz crucible provided in its interior with a space for flowing of a
cooling fluid, which comprises an outer mold section made from a
heat-conductive metal or alloy material and an inner mold section closely
arranged to an inner surface of the outer mold section and made from a
heat-resistant material. [0014](2) A fluid-cooled mold according to the
item (1), wherein vent-holes opening to an inner surface of the mold are
formed in the inner mold section. [0015](3) A fluid-cooled mold according
to the item (1) or (2), wherein the inner mold section is integrally
constituted with a plurality of split segments comprising a dish-shaped
lower split segment placed on a bottom portion of the outer mold section
and at least one ring-shaped upper split segment detachably piled on the
lower split segment and closely arranged to an inner surface of the outer
mold section. [0016](4) A fluid-cooled mold according to the item (3),
wherein each of the plural split segments is made from a carbon material.
[0017](5) A fluid-cooled mold according to the item (1), wherein the
outer mold section is constituted as a fluid-cooling jacket. [0018](6) A
fluid-cooled mold according to the item (1), wherein the mold has a
bottomed cylindrical shape with an upward opening portion and is used for
producing a quartz crucible by a rotating mold method in which the mold
is rotated around a virtual line passing from a central position of the
bottom portion to a central position of the opening portion as an axis of
rotation.

[0019]The fluid-cooled mold according to the invention is a mold for the
production of a quartz crucible, in which the outer mold section
constituting the mold is provided in its interior with a space for
flowing of a cooling fluid and made from a heat-conductive metal or alloy
material and the inner mold section is closely arranged to the inner
surface of the outer mold section and made from a heat-resistant
material, so that raw quartz powder does not directly contact with the
inner surface of the outer mold section. Therefore, even if the mold is
cooled by the outer mold section during the heating at a higher
temperature, the removal of heat from the heated raw quartz powder
through the inner surface of the mold is made less by the heat insulating
effect of the inner mold section made from the heat-resistant material,
and hence the quantity of the raw quartz powder retained at an unmelted
state is reduced significantly.

DESCRIPTION OF THE DRAWINGS

[0020]The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same become better
understood by reference to the following detailed description, when taken
in conjunction with the accompanying drawings, wherein:

[0021]The invention will be described with reference to the accompanying
drawings, wherein:

[0022]FIG. 1 is a longitudinal sectional view of a fluid-cooled mold
according to the invention;

[0023]FIG. 2 is a cross-sectional view illustrating the production of a
crucible by a rotating mold method;

[0024]FIG. 3A is a cross-sectional view showing a state of heating raw
quartz powder when a quartz crucible is produced using a conventional
fluid-cooled mold;

[0025]FIG. 3B is a graph illustrating the temperature of the parts of the
quartz crucible production illustrated in FIG. 3A;

[0026]FIG. 4A is a cross-sectional view showing a state of heating raw
quartz powder when a quartz crucible is produced using the fluid-cooled
mold according to the invention; and

[0027]FIG. 4B. is a graph illustrating the temperature of the parts of the
quartz crucible production illustrated in FIG. 4A.

DETAILED DESCRIPTION

[0028]While illustrative embodiments have been illustrated and described,
it will be appreciated that various changes can be made therein without
departing from the spirit and scope of the invention.

[0029]In FIG. 1 is shown a typical construction example of the
fluid-cooled mold according to the invention. Any fluid known to those of
skill in the art can be used in the disclosed embodiments. In one
embodiment, the fluid is water.

[0030]The illustrated fluid-cooled mold 10 is a fluid-cooled mold for the
production of a quartz crucible and comprises mainly an outer mold
section 11 and an inner mold section 12.

[0031]The outer mold section 11 is provided in its interior with a space
for flowing of a cooling fluid and made from a heat-conductive metal or
alloy material, and there is mentioned a fluid-cooling jacket as an
example thereof. As a material of the outer mold section 11 is preferable
a heat-conductive and corrosion-resistant metal or alloy material having
an excellent cooling ability, which includes, for example, austenitic
stainless steel, atmosphere corrosion resisting steel, titanium and the
like.

[0032]The inner mold section 12 is closely arranged to the inner surface
of the outer mold section and made from a heat-resistant material such as
carbon material, which constitutes the fluid-cooled mold 10 integrally
with the outer mold section 11. Moreover, the fluid-cooled mold 10 has a
bottomed cylindrical shape with an upward opening portion and is used for
producing a quartz crucible by a rotating mold method in which the mold
is rotated around a virtual line passing from a central position of the
bottom portion to a central position of the opening portion as an axis of
rotation.

[0033]The fluid-cooling jacket constituting the outer mold section 11 is
made of stainless steel in which a space for flowing and/or circulating a
cooling fluid is formed inside the jacket and connected to a means (not
shown) for feeding the cooling fluid to the space. To the entire inner
surface of the outer mold section 11 is closely and integrally arranged
the inner mold section 12 made from a heat-resistant material such as
carbon material, for example, a carbon lining.

[0034]In the interior of the inner mold section 12 are formed a plurality
of vent-holes 13 opening to the lining surface. These vent-holes 13 are
communicated with a collective hole 14 formed at a bottom portion of the
mold corresponding to the axis of rotation, and connected to an external
pressure reduction device (not shown) through the collective hole 14. Air
inside a raw quartz powder layer deposited on the inner surface of the
inner mold section 12 is drawn through the vent-holes 13 toward exterior
to hold the raw quartz powder layer at a pressure-reduced state. By
removing air existent in the raw quartz powder layer through suction
during the heat-melting is prevented retention of inner air bubbles in
the vitrification of the quartz powder, whereby a transparent glass layer
substantially containing no air bubble can be formed.

[0035]The inner mold section 12 constituting the fluid-cooled mold 10 may
be detachably made of plural split segments 20 to 24. For example, the
fluid-cooled mold 10 shown in FIG. 1 is integrally constituted with a
dish-shaped lower split segment 20 placed on the bottom portion of the
outer mold section 11 and at least one ring-shaped upper split segment
detachably piled on the lower split segment 20 and closely arranged to
the inner surface of the outer mold section 11, four upper split segments
21 to 24 in FIG. 1. These split segments 20 to 24 form the inner mold
section 12 covering the inner surface of the outer mold section 11.

[0036]In the dish-shaped lower split segment 20 are formed a plurality of
vent-holes 13 passing through the interior and opening at the bottom to
the inner surface of the mold, and a collective hole 14 communicating to
exterior at a center of the bottom portion. Also, the upper split
segments 21 to 23 are ring-shaped members having the same inner and outer
diameters, each of which segments is provided with a vent-hole 13 passing
from an upper surface to a lower surface and opening to the inner surface
of the mold. The uppermost split segment 24 is a ring-shaped member
having the same inner diameter as the other upper split segments 21 to 23
and a U-shaped cross section and provided in its interior with a
vent-hole 13. The uppermost split segment 24 is fixed to an upper end of
the outer mold section 11 through fixing means such as a fixing member 25
engaging with an outwardly opened recess portion of the segment, a bolt
26 and the like. The dish-shaped lower split segment 20 and the upper
split segments 21 to 24 are integrally fixed to each other by the fixing
means, and detachably attached to the inner surface of the outer mold
section 11.

[0037]Thus, the inner mold section 12 is constructed by assembling the
plural split segments 20 to 24, whereby the inner mold section can be
easily attached to the inner surface of the outer mold section 11. Also,
since the inner mold section 12 is detachably attached to the outer mold
section, if the inner mold section 12 is subjected to heat damage, it can
be easily replaced with a new inner mold section. Among the split
segments 20 to 24, only a heat-damaged split segment can be partially
replaced with a new segment, so that the mold maintenance cost can be
kept down but also the service life of the fluid-cooled mold as a whole
can be largely improved.

[0038]As shown in FIG. 2, the fluid-cooled mold 10 according to the
invention has a bottomed cylindrical shape with an upward opening portion
and is rotated around a virtual line passing from a central position of
the bottom portion to a central position of the opening portion as an
axis of rotation to generate centrifugal force, whereby raw quartz powder
is deposited on the inner surface of the inner mold section 12 at a given
thickness by the rotating mold method. Subsequently, the raw quartz
powder is melted by heating to a temperature above a melting point (about
2000° C.) through arc discharge of electrodes 35 disposed above
the mold 10 and around the rotation central axis of the mold to form a
melted quartz layer 34.

[0039]As shown in FIG. 3A, the conventional fluid-cooled mold is not
provided with the inner mold section, so that the raw quartz powder is
directly deposited on an inner surface 31 of the fluid-cooled mold 10 at
the step of heat-melting the quartz powder. The inner surface 31 of the
fluid-cooled mold is cooled to about 100° C., while the inner
surface 32 of the resulting quartz layer 30 is heated to a temperature
higher than a melting point (about 2000° C.), as illustrated in
FIG. 3B. However, since the thermal conductivity of the quartz layer 30
is small, a large temperature gradient occurs in the interior of the
quartz layer 30 ranging from the inner surface 32 of the quartz layer 30
to the inner surface 31 of the fluid-cooled mold 10. As a result, a
portion of the deposited quartz layer corresponding to a temperature
region of not higher than the melting point increases, and hence a ratio
of an unmelted quartz layer 33 occupied in the quartz layer 30 becomes
large. In order to form the melted quartz layer having a target
thickness, therefore, it is necessary to deposit the raw quartz powder at
a thickness taking account of the unmelted quartz layer 33.

[0040]On the contrary, when the fluid-cooled mold 10 comprising the outer
mold section 11 and the inner mold section 12 according to the invention
is used, as shown in FIG. 4A, the raw quartz powder is deposited on the
inner surface of the inner mold section 12 and does not directly contact
with the inner surface of the outer mold section 11. Owing to the
presence of the inner mold section 12, the temperature of the quartz
layer deposited on the inner surface of the inner mold section 12 can be
kept high (as illustrated in FIG. 4B), whereby the thickness of the
unmelted quartz layer 33 left in the quartz layer 30 can be largely
reduced.